Dickens’s Dictionary of the Thames, from its Source to the Nore, an Unconventional Handbook – Charles Dickens, 1885:

A large assemblage of herons takes place at certain times of the year in Richmond Park, where as many as 50 or 60 have been counted at one time. Sometimes they may be seen on the tops of trees, and at others on the ground at a distance from the ponds, appearing perfectly motionless till they are disturbed. This assemblage is very curious. There seems to be no reason why they should congregate and remain for so long a time in the listless manner observed. It is seldom that one sees more than two or three herons together in one place, except at a heronry, and then only when they are watching for their prey.